1. Field of the Invention
The present invention generally relates to an optical line terminal (OLT) installed in a central office that provides services to a lot of optical network units (ONUs) connected with the OLT in a point-to-multipoint network such as the passive optical network (PON), and more specifically to techniques for identifying at least one of the ONUs which transmits data with a different timing than is enabled by the OLT.
2. Description of the Related Art
A passive optical network (PON) is commonly used as one of the technologies to realize a fiber to the home (FTTH) that brings an optical fiber from a central office of, for example, a telecommunication carrier into each customer premise. In the PON, a single optical fiber is branched by using a branching device (e.g., an optical coupler) and brought into a plurality of customer premises. As shown in FIG. 11, the PON includes a point-to-multipoint type structure of an optical subscriber end terminal (i.e., an optical line terminal (OLT)) inside the central office and a plurality of optical network end terminals (i.e., optical network units (ONUs)) that are installed in the customer premises.
As shown in FIG. 11, the OLT transmits a transmission enable data (a data that enables a time period for transmitting data) to each ONU (see (1) of FIG. 11). Each ONU transmits a data to the OLT with timing enabled by the transmission enable data (see (2) of FIG. 11), thus carrying out communication between the OLT and each ONU. As shown in FIG. 11, since data from many ONUs are optically multiplexed in a single optical fiber, if all the data are transmitted with enabled timing, a data collision is not likely to occur (see (3) of FIG. 3).
However, in the PON, because the data (lights) from ONUs are optically multiplexed in a single optical fiber, if the ONUs installed in the customer premises (or the customer premises ONUs) include a defective ONU that transmits data during a disabled time period, a data collision is likely to occur with data that are transmitted from other normal ONUs. The defective ONU must be identified and transmission of data by the defective ONU must be stopped for resuming the communication that is hampered due to the defective ONU.
Various technologies have been suggested for identifying the defective ONU. For example, in a method shown in FIG. 12, upon detecting that data that is transmitted during the time period enabled for an ONU (B) cannot be received (see (1) of FIG. 12), the OLT exercises a transmission stop control to stop data transmission from all the ONUs (see (2) of FIG. 12). Next, the OLT enables data transmission only from an ONU (A) to confirm whether the ONU (A) is operating normally (see (3) of FIG. 12). Next, if the ONU (A) is operating normally, the OLT enables data transmission only from the ONU (B) to confirm whether the ONU (B) is operating normally (see (4) of FIG. 12). Next, if the ONU (B) is operating normally, the OLT enables data transmission only from an ONU (C) to confirm whether the ONU (C) is operating normally (see (5) of FIG. 12). In the example shown in FIG. 12,-because a defect in the operation of the ONU (C) is confirmed, next the OLT stops data transmission only from the ONU (C) and enables data transmission from the other ONU (A) and (B) (see (6) of FIG. 12).
Similarly, in a method that is disclosed in Japanese Patent Laid-open Application No. 2002-359596, upon detecting that data transmitted within a enabled time period cannot be received, first the OLT transmits to all the ONUs, a command that instructs not to carry out allocation of communication bands. Next, the OLT sequentially confirms whether each ONU is operating normally.
However, in the conventional technology mentioned earlier, the communication between the OLT and each normal ONU must be stopped for identifying the defective ONU. In other words, in the conventional technology, in order to identify an defective ONU, the OLT must exercise the transmission stop control to stop data transmission from all the ONUs. Specifically, in a prior system of FIG. 12, identifying an defective ONU, e.g., (C) requires the transmission stop control, causing communication between the OLT and normal ONU (A) and communication between the OLT and normal ONU (B) to be stopped.